The transfer of objects in a marine environment is problematic in that the relative motion engendered between the water's surface and the object being moved must be taken into account and compensated for. Movement of large or heavy objects such as remotely operated vehicles (ROV), vessels and equipment represents a risky endeavor for many reasons, one of which is that large relative degrees of motion are induced into the load due to the ship's response to movement of the water's surface. Docking with or maneuvering near fixed objects, in the ocean or on the ocean floor, while suspended from a ship's crane or other lifting device is nearly impossible unless special means are taken to reduce or eliminate motions induced by the load. Additionally, when the induced motions are in excess of the load's terminal velocity in the water, snap loads occur in the lowering cable. These snap loads are dangerous to the survival of the cable, its terminations, and to the load and lifting device in general. Since these induced motions increase with increasing seas, the range of weather in which these lowering operations can be carried out is restricted. Since larger ships induce smaller motions, larger ships are often required for critical lowering operations in the widest range of weather conditions.
Various heave compensation devices have been proposed in an effort to overcome these difficulties. These devices generally attempt to maintain the load in a more or less fixed position relative to the earth, regardless of the motions that the ship is undergoing. These devices create reciprocating movements in the lowering cable in an attempt to compensate for the relative motion. Control of these devices may be either passive or active, with relative expense being a deciding factor in this regard. Various mechanisms are utilized in order to raise and lower the required amount of cable, including active winch drums, flying sheaves, and nodding booms.